Internet web interface including programmable logic controller for controlling output devices based on status of input devices

ABSTRACT

A control system includes an Internet web interface to a network of at least one programmable logic control system running an application program for controlling output devices in response to status of input devices. The Web interface runs Web pages from an Ethernet board coupled directly to the PLC back plane and includes an HTTP protocol interpreter, a PLC back plane driver, a TCP/IP stack, and an Ethernet board kernel. The Web interface provides access to the PLC back plane by a user at a remote location through the Internet. The interface translates the industry standard Ethernet, TCP/IP and HTTP protocols used on the Internet into data recognizable to the PLC. Using this interface, the user can retrieve all pertinent data regarding the operation of the programmable logic controller system.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/927,005, filed on Sep. 10, 1997. This application is alsorelated to the following, commonly assigned application, entitled"Apparatus for Controlling Internetwork Communications," U.S. patentapplication Ser. No. 08/926,837, filed on Sep. 10, 1997. The contents ofthese applications are expressly incorporated herein by reference.

TECHNICAL FIELD

Applicants' invention relates generally to the field of programmablecontrollers and more particularly to a system for coupling a network ofprogrammable controllers through an internetwork to a monitoring andcontrol device.

BACKGROUND OF THE INVENTION

Remote monitoring and control of systems and processes have taken manyforms. In the past, dedicated lines became the most common form ofcommunication between a control system and a remote location. This haslimited application since the control system was not accessible frommultiple locations. Modems have made it possible to access the controlsystem from different locations, but these types of systems aregenerally restricted to downloading and uploading data files. Providingany type of control function between locations is rather limited in thistype of environment. Further, an end user generally required acustomized interface to access the control system.

With the growth of Internet, and its World Wide Web providing a deliverplatform for organizing Internet data through hypertext links, a clientserver system can be designed that will give each end user the same typeof a user friendly interface with the same universal access to serviceson the Web. The Web is a network of documents called sites or pagesstored on server computers throughout the world Each page will usuallycontain text, some type of multimedia offerings such as graphic images,video, or audio, and possible hypertext links to other documents. Abrowser allows a user to read the pages and interact with the choicesassociated with it. The browser is a graphical software program thatsends commands to the Internet Web site and displays whateverinformation is available on the page. Various browser programs arecommercially available from different manufacturers.

The Internet network employs methods designed to handle thousands ofgeneral purpose computers sharing a single cable, and therefore has noability to differentiate traffic in terms of its purpose or thecriticality of its data. The Internet is no longer a network ofcomputers sharing, a single cable, but rather a web of interconnectedpoint to point links involving both general purpose stations andspecialized infrastructure components such as routers and firewalls.

The type of personal computer or work station used by the end user toconnect to the Web is of no regard. Communication over the Internet andother networks requires one of several types of protocols. Protocolssuch as Internet Protocol (IP) provide for file transfers, electronicmail, and other services. A Sun Microsystem programming language knownas Java, along with Hypertext Markup Language (HTML) used in designinglayouts and graphics for a Web site or page has extended Internettechnology such that a web site can be used for dynamic applications,commonly called applets, that can be downloaded and run by the end user.These applets are interpreted and run within a Web browser and have beengenerally restricted to word processing and similar uses. Downloadingand running applets can be slow in comparison to other types of compiledlanguages. Security rules imposed on a browser and enforced by theunderlying JAVA language prevent applets from obtaining certain datafrom any other device other than the Web server itself.

Programmable logic controllers (PLCs) are widely used in industry andprocess control. Many manufacturers provide factory automationinformation using Microsoft Windows and other types of communicationnetworking environments. These networks are usually slow, are notuniversally accessible and are limited to monitoring and data exchange.Control may be implemented, but since the communication networks arenon-deterministic, control is not real time. Specialized industrialnetworks using proprietary fieldbus alternatives can be very expensive.Conversion products are required to allow information carried over thosenetworks to be visible on a general purpose network. There aresignificant installation and other deployment costs associated with theexistence of such intermediate devices. Firewalls between the Web serverand the application are designed to solve problems of security and arenot designed for high performance.

It would be desirable to develop an automation control system whereby anuser could use general, commercial networks such as the Internet inplace of specialized industrial networks to remotely monitor automationcontrol devices such as PLCs.

SUMMARY OF THE INVENTION

Accordingly, the principal object of the present invention is to providean interface between an industrial control system and a Web browsercoupled to a connectionless network such as Internet.

Another object of the present invention is to provide remote accessthrough a web browser to information and data contained in an industrialcontrol system having a Programmable Logic Controller.

In the preferred embodiment of the invention, the invention allows foreasy access over a commercial network such as Internet to informationwithin a programmable logic controller (PLC). Access can be made locallyor worldwide using a commercial Web browser. The invention is comprisedof a control system of essential elements including, but not limited toa Web interface, a local network, and a network interface to at leastone PLC control system running an application program for controllingoutput devices in response to status of input devices. The Web interfaceruns Web pages from an Ethernet board coupled directly to the PLC backplane and includes an hypertext transfer protocol (HTTP) interpreter, aPLC back plane driver, a Transmission Control Protocol/Internet Protocol(TCP/IP) stack, and an Ethernet board kernel. The Web interface providesaccess to the PLC back plane by a user at a remote location through theInternet. The interface translates the industry standard Ethernet,TCP/IP and HTTP protocols used on the Internet into data recognizable tothe PLC. Using this interface, the user can retrieve all pertinent dataregarding the operation of the PLC, including PLC configuration,Input/Output (I/O) and register status, operating statistics,diagnostics, and distributed I/O configurations. Updates to operatingsoftware can also be downloaded through the Internet access.

Other features and advantages of the invention, which are believed to benovel and nonobvious, will be apparent from the following specificationtaken in conjunction with the accompanying drawings in which there isshown a preferred embodiment of the invention. Reference is made to theclaims for interpreting the full scope of the invention which is notnecessarily represented by such embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an overview block diagram of a typical system illustratingthe relationship between an user at a remote location and an InternetWeb site used for monitoring a process control system according to thepresent invention.

FIG. 2 is a basic block diagram of the present invention illustrating anInternet interface to a programmable logic controller system.

FIG. 3 is a block diagram of the Web server module illustrated in FIG. 2according to the present invention.

FIG. 4 is a typical mimic page available to a user at a remote locationutilizing a browser which illustrates the present invention formonitoring a programmable controller system.

DETAILED DESCRIPTION

Although this invention is susceptible to embodiments of many differentforms, a preferred embodiment will be described and illustrated indetail herein. The present disclosure exemplifies the principles of theinvention and is not to be considered a limit to the broader aspects ofthe invention to the particular embodiment as described.

FIG. 1 shows an overview block diagram of typical system illustratingthe relationship between an user 2 at a remote location and an Internetweb site 4 used for monitoring a process control system 6. The user 2will have a personal computer (PC) 8 having a commercially availablebrowser 10, such as Netscape Communication's Navigator or Microsoft'sInternet Explorer, installed for viewing the contents at the web site 4by a monitor 12 through a network, such as the Internet 14. The PCprovides a remote human-machine interface (HMI) to the process controlsystem 6. Various interconnection services are readily available toprovide the physical and electrical interconnection from the PC to theInternet 14 itself The Internet 14 is a collection of independent worldwide communication networks that are interconnected to each other andfunction as a single connectionless entity. Communication is based on aclient-server basis, using a number of established protocols that allowfor communication and file transfers between the client and the server.The most widely used protocol is Internet Protocol (IP).

The web site 4 includes a network interface 16 having an unique Internetaddress 18, a server 20, and an application program 22. The server 20acts as a hypertext transfer protocol (HTTP) interpreter which usesTransmission Control Protocol (TCP) in conjunction with InternetProtocol, through a Transmission Control Protocol/Internet Protocol(TCP/IP) stack 24 to interact with the network interface 16 and theapplication program 22. This enables the data transfer between theapplication program 22 and the user 2 through the Internet 14. Theapplication program provides data from the process control system 6.This data can be used to monitor the control process by the user 2 atthe remote location. The TCP/IP stack 24 enables data transfers over theInternet 14 between the user 2 and the web site 4 as required for thevarious layers specified by the IP protocol.

The user 2 can connect to the Internet 14 using one of a number ofInternet service providers and will enter the address of the web site 4when connected. The web site 4 will display a home page which maycontain text, some type of multimedia offerings such as graphic images,video, or audio, and possible hypertext links to other documents. Thebrowser 10 will allow the user 2 to read the page and interact with thechoices associated with it. The browser 10 will send commands to the website 4 which will use the application program 22 to display whateverinformation is available from the process control system 6. The browser10 functions as a remote human-machine interface or HMI control of theprocess control system as will be detailed below.

FIG. 2 shows a basic block diagram of the present invention illustratingthe Internet interface to a programmable logic controller system. Theweb site 4 includes the network interface 16 having an unique Internetaddress 18 and a web server 30. The web server 30 provides the home pagefor the website. A firewall or security for the overall system can beincluded in the web server 30, but is generally maintained as part ofthe network interface 16. In addition to providing security for variouspages at the site, the user can disable the web server 30. A passwordand user list is provided in initial configuration files stored in theweb server 30 that are downloaded from a remote server. Protectior ofthe configuration file is then provided by the remote server and the webserver 30 through the password and the user list. The web server 30provides a direct connection for a programmable logic controller (PLC)32 to the Internet 14 by plugging the web server 30 into its back plane34. The web server 30 provides both a client and server interface. Allsignals between the PLC 32 and the web server 30 are through the backplane 34 rather than over a set of cables which would normally have tobe coupled to input/output modules that are themselves plugged into theback plane 34. The back plane signals include addressing, control, data,and power. The client interface allows a user to send commands to aremote node over the Internet and the server interface allows forprocessing commands that originated from a remote node. Controlling thePLC 32 from a remote HMI, essentially on a real time basis is possibleby controlling the data flow through the web server 30.

Associated with the PLC 32 are its application programs 36, dual portmemory 38 and Input/Output (I/O) modules 40. The application programs 36include a ladder logic program for controlling the I/O modules 40. Theweb server 30 functions as a node on a TCP/IP network 42 allowing it tosend commands to the PLC 32 and receive the response. Although theTCP/IP network 42 in the preferred embodiment is an Ethernet network,other high level protocols could be used. Using a web browser at aremote location through the Internet 14, a user can control and viewconfiguration information of the PLC 32.

The web server 30 is shown in greater detail in FIG. 3. Variouscomponents provide the required connectivity to perform itsfunctionality.

A real time operating system 44 controls the interaction between thecomponents. The operating system 44 allocates processor time on acentral processor unit (CPU) 46 to various tasks, provides memorymanagement, and provides a set of message services and signal services.The message and signal services allow for communication between tasks,and between drivers and a task.

Connection to the TCP/IP network 42 is through an Ethernet driver 48which transmits and receives messages at a rate of 100 Mbs (megabits persecond) or faster via an Ethernet communication chip 49, such as, in thecase of 100 Mbs Ethernet, a Crystal CS8952 available from Cirrus Logic,Inc. It is contemplated that in the future Ethernet communication chipscapable of speeds of 1 Gbs (gigabits per second) and faster can be used.The physical connection over which communication is facilitated can beover a fiber optic cable or a twisted pair-type copper wire.

However, the physical connection must be capable of transmitting thesignal at 100 Mbs or faster. In the case of 100 Mbs Ethernet overtwisted pair-type copper wire, the copper wire and wire connections mustconform to at least the Category 5 copper wire standard.

Alternatively, the TCP/IP stack 24 could be replaced by a dual TCP/IPstack. The dual TCP/IP stack comprises a first TCP/IP stack whichprovides support for a broad range of TCP/IP messages. The second TCP/IPstack is a "short stack" which manages high priority control messagesbetween the PLC 32 and I/O modules 40. For outgoing TCP/IP messages, theappropriate TCP/IP stack would be chosen by an application program 36associated with the PLC 32. On incoming TCP/IP messages, the TCP/IPmessage would be intercepted and examined to see if it is a TCP/IPmessage that is intended for the Modbus on Ethernet protocol. If it is aTCP/IP message intended for the Modbus on Ethernet protocol, then themessage is delivered to the second stack. If it is not a TCP/IP messageintended for the Modbus on Ethernet protocol, the first TCP/IP stackhandles the message. In this manner, TCP/IP messages intended for theModbus on Ethernet protocol are handled more quickly and efficientlythan when a single TCP/IP stack is implemented.

The web server will have an unique global address 18, allowing it to beaddressed by other devices on the network. The Ethernet driver 48manages transmit 50 and receive 51 buffers in memory 52, and interfaceswith the Ethernet communication chip 49. The transmit 50 and receive 51buffers are shared both by the Ethernet communication chip 49 and theEthernet driver 48. The Ethernet driver 48 also provides a transmitrequest interface and a receive indication interface to a TCP/IP stack54. The Ethernet communication chip 49 provides a transmit queueinterface, a receive queue interface, and generates interrupts oncompletion of transmitting a message, and on receiving a new message.The Ethernet driver 46 places receive buffers in the receive queue. Inthe interrupt routine, the Ethernet driver 46 examines the receivequeue. If any messages are in the receive queue, it passes the receivebuffer to the TCP/IP stack 54. The TCP/IP stack 54 copies the buffer,and sometime later calls the Ethernet driver 48 to return the buffer andplace the returned buffer back into the receive queue.

The TCP/IP stack 54 calls the Ethernet driver 48 to transmit a message.The Ethernet driver 46 attempts to allocate a buffer from the sharedmemory 52. If it succeeds, it copies the message into the buffer, andplaces the buffer into the Ethernet communication chip 49 transmitqueue. If there is no transmit buffer, then the driver drops thetransmit message. In the interrupt routine, the Ethernet driver 48examines the transmit queue, and frees the transmitted buffers.

The TCP/IP network 42 allows special MSTR (master) functions that allownodes on the network to initiate message transactions. These MSTRfunctions include reading and writing data and are used for commands andresponses. They allow programs running in the PLC 32 to send commands toa remote node on the TCP/IP network 42 and receive the responses. A backplane driver 56 sends commands and receives the response to the PLC 32over the back plane 34.

The back plane driver 56 receives request from the PLC's ladder logicMSTR blocks stored in its memory 38. When a response is available, theback plane driver 56 passes it back to the MSTR block. The back planedriver 56 provides a client task 58 and server task 60 to theapplications. The server task 60 interface allows an application toissue a request command to the PLC's 32 executive program and receiveits response. The client task 58 allows an application to receive a newMSTR request and pass back the response to the ladder logic program.

The server task 60 uses a queuing mechanism and call back functions. Anapplication queues both the request and the call back functionassociated with the request. When the back plane driver 56 services therequest in its interrupt routine, it calls the associated call backfunction. The response and the original request is passed to the callback function. The call back function can call an operating routine toeither pass a message or signal the application.

The client task 58 also uses queues and call back functions. The clientapplication queues both an indication request on queue and a call backfunction associated with the request. When the back plane driver 56detects a new MSTR block request in its interrupt routine, it calls theassociated call back function. The request is passed into the call backfunction. The call back function can call an operating system routine toeither pass a message or signal the application. If the back planedriver 56 detects that the MSTR block has been aborted, or is no longerbeing solved, it calls an user supplied associated abort call backfunction The application calls a routine to pass the MSTR response andan associated call back routine to the driver. Sometime later, thedriver passes back the response to the ladder logic program in itsinterrupt service routine, and then calls the user supplied call backfunction.

The PLC 32 interfaces with the web server 30 hardware via the dual portmemory 38. It reads and writes to the dual port memory 38 using an ASICchip. Writing to a specified location will cause an interrupt. The PLC32 first writes a message in the dual port memory 38, and then causes aninterrupt. The message indicates a type of command. One type indicatesthat a MSTR block is being solved. Other types are used for passingrequests to the PLC 32 and obtaining the responses to the requests.After the PLC 32 passes the message, it polls the dual port memory 38for commands placed by the back plane driver 56. These commands are readmemory, write memory, and processing is complete. The back plane driver56 uses state machines to process the MSTR interrupts. The maximumnumber of active MSTR blocks is set at four in the present invention,requiring four state machines. When the back plane driver 56 receives aMSTR interrupt, it attempts to find an associated state machine thatmatches with the MSTR block. If there are already four outstandingtransactions, no more are available, and the back plane driver 56 willset the MSTR's outputs to false. If a state machine is found, the backplane driver 56 determines if it is a new transaction, an outstandingtransaction, or a response is available. If it is a new transaction itcopies the request and calls the application's associated call backroutine. If its an outstanding transaction, it indicates to the ladderlogic program that the MSTR block is still busy. If a response isavailable, the back plane driver 56 copies the response, sets either theMSTR's completion or error output, and calls the application's call backroutine.

Two interrupts are used for processing a request. On the firstinterrupt, called the preport interrupt, the back plane driver 56 copiesthe request into a data structure located in the PLC's 32 dual memory38. On the second interrupt, called the end of scan interrupt, the backplane drive 56 copies the response from the controller's data structureinto the user's buffer. It then calls the user's associated call backfunction.

The request for accessing the PLC's 32 registers is processed by theback plane driver 56 and is not sent to the PLC's executive pro-ram forprocessing. The back plane driver 56 determines the memory location inthe memory 38 of the registers the PLC 32. At an end of scan interrupt,the back plane driver 56 processes the read/write register requests bysending commands via the dual port memory 38 to the PLC 32 to read orwrite the locations containing the registers. The back plane driver 56will service a maximum of four read/write register requests at the endof a scan interrupt.

A client task 58 interfaces with the TCP/IP stack 54, the back planedriver 56 and uses the operating system 44 message services. Itprocesses the MSTR request. When the client task 58 receives a MSTRrequest from the back plane driver 56, it passes the request to theTCP/IP stack 54. When the TCP/IP stack 54 returns a response to theclient task 58, it passes the response to the back plane driver 56. TheTCP/IP stack 54 provides a Berkeley TCP/IP interface and a signalextension. The signal extension calls a user supplied function whichpasses in a socket number, a task ID, and an event. The signal functioncalls the operating system 44 to send a message to the task indicated bythe task ID. It sends a message either to the client task 58 or servertask 60. The client task 58 posts request indications to the back planedriver 56, and the associated call back routine calls the operatingsystem 44 to send a message to the client task 58 for a new MSTRtransaction.

The client task 58 manages multiple outstanding MSTR transactions usingthe state machines. There is a linked list of connection state machines.The connection state machines are used for establishing connection andclosing connections. In addition each connection state machine containsa list of transaction state machines. Each transaction machine on theconnection state machine represents a transaction to a node representedby the connection machine. The transaction machines are used to send arequest, and process the response. The client task 58 enters a loopafter performing initialization. It calls the operating system 44 toreceive a message. The operating system will block the client task 58until there is a message or until there is a time out. It eitherreceives a message from the TCP/IP stack 54, from a MSTR call backroutine, or it times out. It process the message or the time out andthen reenters the loop. If the message received from the operatingsystem 44 is a new MSTR request, the client task will obtain aconnection state machine, and places a new transaction machine at end ofthe list of the connection state machine's list. At this point thetransaction machine will attempt to transmit the message. It may not bepossible to transmit the message because no connection has beenestablished, or because the remote side may be applied flow control.

If the message received from the operating system 44 is a TCP/IP event,the client task 58 finds the associated connection machine anddetermines if the TCP/IP event is an accepted connection, an abortedconnection, or a received data event. Based on the connection state, andthe transaction machine's state, the client task 58 processes themessage to advance the transactions if there are any. Receiving data forthe MSTR responses may occur over several TCP/IP events, and thetransaction state machine assembles the data into a response. When theclient task 58 requests the TCP/IP stack to transmit a message, not allof the message may be transmitted. This occurs when the remote node isflow controlled, which is explained below. If the call to the operatingsystem 44 to receive a message returns with a time out, or if there is amessage, the client task 58 searches the list of connection machinesthat are flowed controlled. For each flow controlled connection, ittries to advance the transaction state machines on the connection statemachine list that are flow controlled.

The server task 60 processes a request originating from the user at theremote location. The server task 60 interfaces with the back planedriver 56, the TCP/IP stack 54, and the operating system's 44 messageservices. The server task 60 posts requests to the back plane driver 56,and an associated call back routine uses the operating system 44 messageservices to send the response to the server task 60. A TCP/IP stack 54signal function also uses the operating system's 44 send service to sendan TCP/IP event to the server task 60. The server task 60 can handlemultiple transactions and connections. Like the client task 58, itmaintains a list of connection machines, and each connection machinecontains a list of transaction machines. The connection machines are formanaging the connection and the transaction machines manage the incomingrequests and responses.

The server task 60 enters a loop after performing initialization. Itcalls the operating systems 44 to receive a message. The operatingsystems 44 block the server task 60 until there is a message or until ittimes out. It either receives a message from the TCP/IP task's 54 signalhandler, from the back plane driver 56 or it times out. It processes themessage or the time and reenters the loop. If the message received fromthe operating systems 44 is from the TCP/IP task's 54 signal handler,the server task 60 determines if the event is a connection request, aclose socket event, or a receive data event. Based on the TCP/IP event,the server task 60 uses the connection machine and transaction machineto advance the transaction. Received data for a request may occur overseveral receive data events, and the transaction machine assembles theevents into a request message. When the response message is receivedfrom the operating system 44, the server task 60 finds the connectionand transaction machine in order to send the response.

When the server task 60 requests the TCP/IP stack 54 to transmit amessage, not all of the message may be transmitted. This occurs when theremote node is flow controlled. If the call to the operating system 44is to receive a message returns with a time out, or if there is amessage, the server task 54 searches the list of connection machinesthat are flowed controlled. For each flow controlled connection, ittries to advance the transaction state machines on the connection statemachine list that are flow controlled.

After the server task 60 has parsed the header of an incoming request,it attempts to allocate a structure to pass the request to the backplane driver 56. If the server task is already processing apredetermined number of outstanding requests, the attempt fails, theconnection is placed into a blocked state, and the body of the requestis not read from the TCP/IP stack 54. As a result the TCP/IP stack mayapply flow control to the remote node. When one of the other requests iscomplete, the free data structure event causes a blocked connectionmachine to continue processing the incoming Modbus request.

The HTTP task 62 interfaces with the TCP/IP stack 54, and the back planedriver 56. The HTTP server task 62 receives a HTTP request from theTCP/IP stack 54. To process the request, it may access the PLC 32through the back plane driver 56 and back plane 34. The HTTP server task62 sends back the response over the TCP/IP stack 54. The framework issupplied by the operating system 44. The framework creates the HTTPtask, accepts connection, and parses the HTTP request. After parsing therequest, it calls the operating system 44 to process the request.Processing the request involves determining the request type andprocessing the actual request. The different request types allow a userto acquire a snapshot of the PLC 32 operations by allowing a view ofvarious registers within the PLC 32 and dual memory 38. These requesttypes also include display of the PLC 32 configuration, remote anddistributed I/O and module health statistics, display registers, backplane configuration, Ethernet statistics and others, as shown in Table1:

                  TABLE 1                                                         ______________________________________                                        Show the home page                                                              Show the programmable logic controller's configuration                        Show the Ethernet statistics                                                  Show the read register request page                                           Show the 4x registers                                                         Show the racks attached to the controllers back plane                         Send an image. The different images are gif files that are                    displayed on the various pages                                                Show the remote I/O statistics                                                Show the list of configured remote I/O drops                                  Show a remote I/O rack's configuration and health                             Show a remote I/O drop's communication statistics                             Show the I/O reference values of a remote I/O module                          Show a list of configured distributed I/O nodes                               Show the configuration and the health of a distributed I/O node              Show the I/O reference values of a distributed I/O module                    ______________________________________                                    

The home page contains hyperlinks to seven pages of data. Theconfiguration page will display the configuration of PLC 32. The remoteI/O and distributed I/O module health status pages are a series oflinked pages. The first page displays the communication healthstatistics at the Remote I/O and Distributed I/O head and contains alink to a configured drop page. The configured drop page displays atable containing drop numbers which are linked to a drop status page andrack numbers which are linked to the drop and rack configuration pages.Two tables are included in the drop status page, one for showing thecommunication status of the drop and the other for showing which racksare populated with the I/O modules. The drop and rack configuration pagedisplays the I/O modules, their health, and slot location for the givenrack. From a selected module, a user can view it's input and outputvalues. Register data is displayed in a template having a form and atable, with the user entering an address and a length. The table willdisplay the register's values A table showing option modules and theirslot location is displayed on the back plane configuration page. Thedata appearing on the pages is static but can be automatically updatedat preselected times.

The operating system 44 processes these requests and responds by sendingHTTP messages through the TCP/IP stack 54. Processing some of theserequests involves reading the PLC's traffic cop, registers, coils, orvarious page zero locations where statistics are kept. To perform thesereads, the operating system 44 sends a request to the back plane driver56 and uses an event signal mechanism and event flags to determine whenthe request is complete. After sending the request to the back planedriver 56, the operating system 44 waits for an event flag to be sent.When the back plane driver completes the request, the back plane driver56 calls a call back routine, which sets the event. The operating system44 then resumes processing the request.

A mimic page which represents some of the hardware physically connectedto a programmable logic controller system can be constructed utilizingvarious graphical programs readily available and that are not an objectof the present invention. The present invention allows a user at aremote location, using a browser, to view the mimic page and actuallycontrol various components illustrated in the mimic page. FIG. 4 shows asimple motor start-stop control in ladder logic diagram form that couldbe available as a mimic page to the user. Pushing a motor start pushbutton 150 will cause a motor start relay 152 to energize through anormally closed stop push button 154 and a normally closed overloadcontact 156. Auxiliary motor start contact 158 will latch relay 152after the start push button 150 is released and pilot light 160 willilluminate. Auxiliary motor start contact 162 will provide power to pumpmotor 164 which will remain running until stop push button 154 isdepressed or overload relay 166 detects an overload condition. In thisexample, start push button 150, stop push button 154, overload contact156, auxiliary motor start contacts 158 and 162, and overload relay 166are inputs to the programmable logic controller system. Relay 152, pilotlight 160, and pump motor 164 are outputs. The PLC will have theregisters containing the animation data for the inputs and outputs. Anapplication program in the PLC will respond to the inputs to control theoutputs

A user at a remote location will browse the Internet for the home pageof the installation of the programmable logic controller system. The PLCwill have other control functions as well and if the user has thenecessary authorizations, various options will become available. Thehome page will allow the user to acquire a snapshot of the PLCoperations by allowing a view of various pages that will allow access toregisters within the PLC. Other pages will also include displays of thePLC's configuration, remote and distributed I/O modules healthstatistics, display registers, back plane configuration, Ethernetstatistics and others as shown previously shown in Table 1.

The mimic diagram page will be called up on a browser screen which willallow the user to view the status of the system. The mimic diagram'slight 160, relay 152, contacts 158, 162, and pump motor 164 will beupdated to correspond to the state of the actual devices. The states ofthe inputs and outputs will then be shown on the ladder diagram whichwill be automatically updated as they are changed. Through the use ofapplets representing the start 150 and stop 154 buttons, the user couldmanually control start and stopping of the motor by using a mouse orkeyboard to position a cursor and "clicking" on either the start 168 orstop 170 boxes.

While the specific embodiments have been illustrated and described,numerous modifications are possible without departing from the scope orspirit of the invention.

We claim:
 1. An interface module for receiving data requests from aremote location in order to provide access to a programmable logiccontroller from a communication network, the module comprising:a centralprocessing unit; a real time operating system operating the centralprocessing unit; a network interface for communicating with thecommunication network at a nominal data transfer rate of not less than100 Mbs; a back plane driver for communicating with the programmablelogic controller through a back plane of the programmable logiccontroller; a protocol stack; a client task for communicating with theback plane driver and the protocol stack for initiating receivedrequests; a server task for communicating with the back plane driver andthe protocol stack for responding to received requests; and a protocoltask for communicating with the back plane driver and the protocol stackfor receiving and responding to protocol task requests.
 2. The interfacemodule of claim 1 wherein the communication network is a world-widenetwork known as the Internet using an Internet Protocol (IP).
 3. Theinterface module of claim 2 wherein the interface module functions as aweb site on the Internet, the interface module including a global IPaddress.
 4. The interface module of claim 1 wherein the networkinterface is an Ethernet driver.
 5. The interface module of claim 1wherein the protocol stack is a Transmission Control Protocol stack. 6.The interface module of claim 1 wherein the protocol task includes aserver task using a hypertext transport protocol (HTTP) to deliverhypertext documents to the network interface.
 7. The interface module ofclaim 6 wherein the HTTP task accepts a connection, parses an HTTPrequest, and calls the real time operating system to process therequest.
 8. The interface module of claim 2 wherein the interface moduleallows a user at a remote location to view data within the programmablelogic controller from a browser connected to the Internet.
 9. Theinterface module of claim 1 wherein the data transfer enabling meansincludes a dual TCP/IP stack.
 10. The interface module of claim 9wherein the dual TCP/IP stack comprises a first stack capable ofhandling a broad range of TCP/IP messages and a second stack capable ofhandling a less broad range of TCP/IP messages more quickly than thefirst stack.
 11. An interface module for receiving data requests from aremote location in order to provide access to a programmable logiccontroller from a communication network, the module comprising:a centralprocessing unit; a real time operating system operating the centralprocessing unit; a network interface for communicating with thecommunication network; a back plane driver for communicating with theprogrammable logic controller through a back plane of the programmablelogic controller; a dual protocol stack; a client task for communicatingwith the back plane driver and the protocol stack for initiatingreceived requests; a server task for communicating with the back planedriver and the protocol stack for responding to received requests; and aprotocol task for communicating with the back plane driver and theprotocol stack for receiving and responding to protocol task requests.12. The interface module of claim 11 wherein the communication networkis a world-wide network known as the Internet using an Internet Protocol(IP).
 13. The interface module of claim 12 wherein the interface modulefunctions as a web site on the Internet, the interface module includinga global IP address.
 14. The interface module of claim 11 wherein thenetwork interface is an Ethernet driver.
 15. The interface module ofclaim 11 wherein the protocol stack is a Transmission Control Protocolstack.
 16. The interface module of claim 11 wherein the protocol taskincludes a server task using a hypertext transport protocol (HTTP) todeliver hypertext documents to the network interface.
 17. The interfacemodule of claim 16 wherein the HTTP task accepts a connection, parses anHTTP request, and calls the real time operating system to process therequest.
 18. The interface module of claim 12 wherein the interfacemodule allows a user at a remote location to view data within theprogrammable logic controller from a browser connected to the Internet.19. The interface module of claim 11 wherein the dual protocol stackcomprises a dual TCP/IP stack.
 20. The interface module of claim 19wherein the dual TCP/IP stack comprises a first stack capable ofhandling a broad range of TCP/IP messages and a second stack capable ofhandling a less broad range of TCP/IP messages more quickly than thefirst stack.